Regarding printers for printing onto a printing medium, there have conventionally been known printers that employ a thermal head as printing means thereof, as well as ink-jet printers and laser printers. The printers with a thermal head are easy for miniaturization and price-reduction in comparison with the ink-jet printers and the laser printers. Therefore, the printers with a thermal head are used for tape printing apparatuses so as to print letters and figures on a tape fed from a tape cassette housed therein, for instance.
As variations of printers with a thermal head as printing means, there have been used heat-sensitive printers that carry out printing onto heat-sensitive paper and thermal transfer printers wherein, for printing, ink in an ink layer of an ink ribbon is transferred onto a printing medium by using a thermal head. Particularly, the thermal transfer printers are superior to the heat-sensitive printers in terms of that quality of printing by the thermal transfer printers is hard to deteriorate even after passage of long time in comparison with quality of printing by the heat-sensitive printers and that discoloration of a printing medium can be avoided in case of thermal transfer printers.
Furthermore, there has conventionally been required for the printers with a thermal head to print quickly so as to shorten printing time. Thermal transfer printers, however, have caused problems as will be described below in case of quick printing.
FIG. 11 shows an example of an energization waveform with respect to a heater element of a thermal head in a thermal transfer printer and a heating pattern thereof. A heat transfer printer includes a thermal head, used as printing means, consisting of a plurality (e.g. 128 or 256) of heater elements aligned crosswise with respect to a conveying direction of a printing medium. Once printing is started, one line of printing data (line printing data) is transferred to the thermal head from a control unit. Thereafter, heater elements to be used for printing based on the transferred printing data are electrically energized as the waveform shown in FIG. 11 indicates. It is to be noted that an energization waveform consists of: “preheating 1” for compensating thermal capacity shortage of a thermal head at initial stage of printing; “preheating 2” for raising temperature up to predetermined temperature (referred to as ink-melting temperature, hereinafter) so that a heater element to be used for printing is heated enough for thermal transfer (i.e., temperature hot enough to melt an ink layer of an ink ribbon); and “heating” for constantly keeping temperature of the heater element to be used for printing at the ink-melting temperature). Thermal transfer based on one line of printing data is carried out as single printing cycle.
Heater elements are energized, as the waveform of FIG. 11 indicates, whereby the heater elements are heated up to the ink-melting temperature or higher and ink of an ink layer is transferred onto a printing medium in dot shape with respect to each of the heated heater elements. By repeating the above-described one line of thermal transfer and conveyance of the printing medium, desired letters and figures are printed on the printing medium. For improving printing speed, it is necessary to shorten a printing cycle of one line of printing, in other words, shorten time to energize a heater element.
However, shortening of energization time means that the same heat quantity has to be applied to a heater element within a short time. This manner has required high power and increased burden to a CPU. Since printers with a thermal head have often been adapted to the as-mentioned compact and simple structured apparatuses, high-powered design and installation of a high performance CPU have been hard to put into practice. There has been devised a conventional heat transfer printer that thins out some dots for quick printing.
In the conventional printer, quick printing is realized by thinning out some dots, i.e., reducing the number of lines for printing. Therefore, an empty space arises between printed dots 151 as shown in FIG. 12. Consequently, edge portions of printed letters and figures are considerably rough, which degrades printing quality.